Measurement of colour flow using jet-pull observables in $t\bar{t}$ events with the ATLAS experiment at $\sqrt{s} = 13$ TeV

Previous phenomenological studies and measurements have shown that weighted angular moments derived from jet constituents encode the colour connections between initiating partons which seed the jets. This note presents measurements of two such distributions, the jet-pull angle and jet-pull magnitude, both of which are derived from the jet-pull-angular moment. The measurement is performed in $t\bar{t}$ events with one leptonically decaying $W$ boson and one hadronically decaying $W$ boson using $36.1\,\text{fb}^{-1}$ of $pp$ collision data recorded by the ATLAS detector at $\sqrt{s} = 13 \, \text{TeV}$ delivered by the Large Hadron Collider. The observables are measured for two di-jet systems, corresponding to the colour-connected daughters of the $W$ boson and the two $b$-jets from the top quark decays. To allow the comparison of the measured distributions to colour model predictions, they are unfolded to particle level, thereby removing experimental effects introduced by the detector. In general, the observables sensitive to colour flow remain poorly modelled by MC predictions. While good agreement can be found for individual prediction and observables combinations, none of the predictions describes the data well across all observables.